It is known to incorporate an afterburner in the exhaust system of an internal combustion engine in order to heat a catalytic converter so as to reduce the time it takes to reach its light-off temperature. The flame in the afterburner heats up the front face of the matrix in the converter but the heat is localised to this area for some time. As this portion of the matrix is subjected to the most severe conditions, its catalyst is the most prone to contamination and therefore when the catalytic converter ages, raising the temperature of its front face does not have the desired effect of reducing the time taken for it to become effective.